1. Field of The Invention
The present invention relates to an apparatus and method for detecting intake air quantity for internal combustion engine which compensate for a response delay of detection at a time immediately after a power supply is turned on to supply a power to a temperature sensitive airflow meter detecting the intake air quantity sucked into internal combustion engine on a basis of a resistance change in a temperature-sensitive resistor disposed in an intake air passage of the internal combustion engine.
2. Description of The Background Art
In an electronic controlled fuel injection apparatus installed in an internal combustion engine, an airflow meter to detect an intake air quantity Q of the engine is provided and a basic fuel injection quantity Tp is calculated as Tp =K.times.Q/N (K denotes a constant) from the intake air quantity Q detected by the airflow meter and an engine revolution speed N.
A temperature-sensitive airflow meter used in the above-described electronically controlled fuel injection apparatus is used which is disclosed in a Japanese Utility Model Registration Application First Publication No. Showa 59-78926.
The temperature-sensitive airflow meter includes a hot-wire type or hot-film type temperature sensitive resistor disposed in an intake air passage. A current is supplied to the temperature-sensitive resistor to generate heat on the temperature-sensitive resistor towards a constant temperature (resistance value). A temperature reduction due to an exposure to the intake air is compensated for by an increase in the supply current, deriving the intake air quantity from the current value.
Although a structure of the temperature-sensitive flow meter will be described in the DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT with reference to FIG. 2, the temperature-sensitive airflow meter will be explained below:
A bridge circuit B is constituted by a temperature compensation resistor R.sub.k, reference resistor R.sub.s, and fixed resistors R.sub.1 and R.sub.2 in addition to the temperature sensitive resistor R.sub.H (hot-wire or hot-film) .
A differential amplifier OP has positive and negative input terminals to which a junction between the temperature compensating resistor R.sub.k and R.sub.1 and first resistor R.sub.2 is connected to receive a terminal voltage across the fixed resistor R.sub.2 and to which a junction between the temperature-sensitive resistor R.sub.H and reference resistor R.sub.s is connected to receive a terminal voltage across the reference resistor R.sub.s, respectively.
A supply current to the bridge circuit B via a transistor Tr is corrected according to an output off the differential amplifier OP.
In detail,s, when the intake air quantity of the engine is, for example, increased with the bridge circuit B in an equilibrium state, the temperature-sensitive resistor R.sub.H is cooled by its air stream passing therethrough so that its resistance value is accordingly reduced and a terminal voltage across the reference resistor R.sub.s is increased so that the bridge circuit B is not in the equilibrium state . Consequently, the output current of the differential amplifier OP is increased . The supply current to the bridge circuit B is thereby increased through the transistor Tr so as to heat the temperature sensitive resistor R.sub.H. Therefore , the resistance value of the resistor R.sub.H is increased so that the equilibrium state of the bridge circuit B is recovered.
For example, when a temperature of the intake air is reduced, the temperature-sensitive resistor R.sub.H is cooled and its resistance value is reduced. However, since the temperature compensating resistor R.sub.K which is in the same atmosphere as the temperature-sensitive resistor R.sub.H is simultaneously cooled and its resistance value is also reduced. Consequently, a change in the current value supplied to the bridge circuit B due to the change in the temperature of the intake air is suppressed.
Hence, the intake air quantity of the engine and supply current to the bridge circuit B correspond to each other irrespective of the temperature of the intake air. Consequently, the intake air quantity can be measured by detecting the terminal voltage across the reference resistor R.sub.s.
As described above, the control of the supply current is carried out so as to maintain the temperature of the temperature-sensitive resistor at a constant in the temperature-sensitive type flow meter using the temperature-sensitive resistor. Therefore, a predetermined period of time is required according to the thermal capacity of the temperature-sensitive resistor in order for the temperature-sensitive resistor in an ambient temperature state to reach a normal control temperature (,e.g., approximately 400.degree. C.) upon a turn on of a power supply to the bridge circuit B.
Especially, the temperature sensitive airflow meter of the hot-film type generally having a large thermal capacity requires a relatively long term until reaching the normal control temperature, as compared with the hot-wire type.
During the time when the temperature of the temperature-sensitive resistor reaches from the at the time when the power supply is turned on to the proximity to the normal control temperature, the bridge circuit: B is not in the equilibrium state so that a large current is supplied thereto in order to increase the temperature of the temperature-sensitive resistor. The large current at that time is not caused by the temperature reduction in the temperature-sensitive resistor due to the increase in the intake air flow quantity but is required to increase the temperature of the temperature-sensitive resistor from the ambient temperature level to the proximity to the normal control temperature level. Consequently, during the time at which the temperature of the temperature-sensitive resistor increases from that immediately after the power supply is turned on to the proximity to the normal control temperature, the intake air quantity cannot accurately be detected, as the actual matter of fact, and the detected intake air quantity is largely deviated from a true intake air quantity.
Thus, if the engine is in a start condition until the temperature of the temperature-sensitive resistor reaches the vicinity to the normal control temperature, the basic fuel injection quantity Tp is calculated on the basis of a larger air quantity than the true (real) intake air quantity. Consequently, an air-fuel mixture ratio of an air mixture fuel sucked to the engine is enriched and an ill influence of the rich air/fuel mixture ratio may be given to an engine start characteristic and exhaust gas characteristics.
Furthermore, as shown in FIG. 2, an error characteristic is varied according to the passed time t upon the power supply turn on. Therefore, the influence on the start characteristic is varied depending on the time duration from the time at which the power supply is turned on to the time at which the engine start is carried out.
It is noted that FIG. 2 shows a result of experiment in which the temperature-sensitive resistor is disposed in an air stream having a constant flow quantity and the detection error is monitored which is decreased with the increase in the temperature of the temperature-sensitive resistor upon the power supply turn on
FIG. 2, in other words, shows a static error characteristic which accords with the passed time described above.
A Japanese Patent Application No. Heisei 3-312452 exemplifies a previously proposed temperature-sensitive airflow meter in which the detection error during the time duration from the time at which the power supply is turned on to the time at which the temperature of the temperature-sensitive resistor reaches the normal control temperature is simulated on the basis of the passed time upon the turn on of the power supply to the temperature sensitive airflow meter and a characteristic such that the output voltage of the temperature sensitive airflow meter is converted into the data of the intake air flow quantity is corrected on the basis of the passed time.
However, if the intake air quantity is changed during the time duration at which the temperature of the temperature-sensitive resistor does not reach the normal control temperature, the value of supply current to the temperature-sensitive resistor is different from that at the normal control time and, therefore, a responsive characteristic in the detection signal with respect to the change in the air flow quantity is different. In general, it was indicated that a detection response delay occurred due to a large temperature difference of an initial time at which the power supply was turned on from that at the time when the normal control temperature was reached.
Hence, in the case of the previously proposed airflow meter correction structure described above, when the intake air flow quantity is constant (in the static state), a desired correction can be made. However, when the intake air flow quantity is changed (in a dynamic state), a large response delay occurs as denoted by a dotted line shown in FIG. 1.
It is noted that a solid line shown in FIG. 1 denotes the true intake air quantity change and a do-and dash line denotes a normally detected intake air quantity with the temperature of the temperature sensitive resistor being the normal control temperature. Consequently, only in the correction control based on the passed time described above, it is impossible to compensate for the detection characteristic with a high accuracy from the time when the power supply is turned on to the time at which the temperature of the temperature-sensitive resistor reaches the normal control temperature.